The present invention relates to a cable pulling/feeding apparatus used when a cable is to be installed or manufactured.
Cables are normally installed through a tube or the like by a cable pulling unit. When the cable is subjected to an excessive tension while being pulled by the cable pulling unit, the cable is adversely affected by such an undue tensioning force. For optical fiber cables, extremely longer lengths of optical fiber cable as compared with conventional metal cables are installed to reduce splice losses. When such an optical fiber cable is tensioned excessively, it can be elongated or bent and will cause an optical transmission loss. Various protective measures have been proposed to prevent such an undue tension from being imposed on the cable. One such proposal enables the cable pulling unit to rotate idly when the tension applied to the cable exceeds an allowable level. According to another prior effort, an electric current to be supplied to a drive motor in the cable pulling unit is limited to a predetermined value so that the drive motor will not produce a drive force greater than a certain level. These conventional arrangements thus detect and control the torque of the cable pulling unit mechanically or electrically. However, where the power transmission mechanism is complicated, the mechanical loss is large in proportion and the correct tension can not be measured, with the result that the correct torque control is difficult to achieve.
There is known a method of measuring the tension of a wire by detecting a displacement of a movable roller held against the wire to cause the latter to travel along a curved path. This method is disadvantageous in that the wire undergoes unwanted pressure from the movable roller and is subjected to resistance to travel by the movable roller.